Field of the Invention
The invention relates to a method for balancing the electrical voltages of at least two serially connected electrical accumulator units. The invention also relates to a corresponding electrical accumulator.
Description of the Prior Art
It is clear that in future, in both stationary applications, such as wind farms and non-stationary applications, such as in vehicles, for example hybrid and electric vehicles, new battery systems of which very stringent demands for reliability will be made will increasingly come into use. The background of these demands is that a failure of the battery systems can lead to either a failure of an entire system pertaining to the application, or to a safety-relevant problem. One conceivable example of such a failure is an electric vehicle that if its traction battery fails is “dead in the water”, since it is no longer capable of propelling itself. As an example of a safety-relevant problem, a wind farm is conceivable, in which electrical accumulators are used for protecting the farm against impermissible modes of operation by adjusting the rotor blades under strong wind conditions. Failure of these electrical accumulators can then lead to safety-relevant problems.
When many individual accumulator units, such as battery cells, connected in series are used, the individual accumulator units are not automatically equal. As a result, particularly over the service life of the accumulator units, this leads to unequal electrical voltages among the individual accumulator units, unless appropriate countermeasures are taken. Especially with lithium-ion batteries, excessive charging or deep discharging of individual accumulator units leads to irreversible damage. Such excessive charging or deep discharging can result when a battery management system regulates a charging or discharging operation based on one of the accumulator units, which is not representative all of the accumulator units. For that reason, balancing of the electrical voltages of the electrical accumulator units among one another must be done at regular intervals. This balancing is known as “cell balancing”. To that end, the individual accumulator units are discharged, by external wiring provisions, in such a way that after the balancing, they all have the same electrical voltage.
It is known for that purpose to perform so-called resistance balancing. To that end, an ohmic resistor or a resistor combination is assigned to each accumulator unit via switches. By means of the resistors, the accumulator units are discharged until such time as the accumulator units have the electrical voltage. It is disadvantageous here that energy stored in the electrical accumulator units is converted into heat by the resistors and is carried away unused, for the sake of achieving the desired charge balance. Hence there is a need for a way in which balancing the electrical voltages of a plurality of accumulator units among one another is attained with little energy loss and in which a substantial improvement in the efficiency of a complete electrical accumulator system is brought about.